Stepping motors are widely used for moving the read/write head of a disk storage device in which the rotor or a movable piece of the stepping motor is driven one step each time a drive instruction is provided. In that sense, a stepping motor may be regarded as a type of pulse motor, but it is more typically a polyphase motor, ordinarily of two-phase construction. Phase data designating the phase current vectors of the stepping motor are provided to a drive circuit for supplying the respective phase currents to the stepping motor. The phase data is switched to sequentially designate the positions of the rotor or the movable piece, thereby shifting the movable piece by a desired amount or step number.
By providing data designating the phase current vectors of the stepping motor to the drive circuit, it is possible to stop the movable piece of the stepping motor at a desired position. By controlling the timing for switching the phase data to the drive circuit, it is further possible to drive the stepping motor according to a desired schedule.
In order to move the read/write head desired distance within the shortest possible time, the phase data given to the drive circuit of the stepping motor is switched at times so that head speed changes in an approximate form of trapezoid or triangle. For that purpose, the time interval for switching the phase data is progressively shortened during the acceleration period, kept constant during the constant speed period, and progressively prolonged during the deceleration period. The schedules for contracting and expanding the switching interval are usually determined from the number of steps. The time interval between switching can be determined from the desired speed and the time characteristic of the read/write head.
The drive object, such as a read/write head, necessarily has inertia, however, so that when the time for driving the object is reduced below a certain limit, the actual displacement of the object tends to deviate from the driving schedule. In particular, the object may not be brought to a stop until after passing the desired stopping position. The object then oscillates around the desired stopping position. In the stepping motor, the position of the movable piece is designated by the phase data in the drive circuit, so that the movable piece is eventually brought to a stop at the designated phase current position. When the moving piece oscillates around the stopping position, however, additional time is required for the oscillation to subside.
Various techniques have been used to damp this oscillation. For example, so-called "anti-phase damping" is one such technique. When the object approaches the final stopping position, the phase data given to the drive circuit of the stepping motor is not switched to the phase data designating the stopping position, but instead is switched to a phase data that is two or three steps previous in order to apply a damping force to the object. The phase data is reswitched to the normal phase data at an appropriate time. Although this technique can be quite effective, getting the timing right for the "anti-phase" switching and the timing for reinstating the normal phase data is difficult, as one can easily imagine. Furthermore, the timing will vary with the conditions such as the driving distance which may actually encourage the oscillation.